I don't have such a directory; /usr/share/icons/hicolor/ contains only:
a bunch of nnxnn/ directories
scaleable/
scalable/
icon-theme.cache
index.theme

Each of those directories seems to have one or more of apps/, devices/, filesystems/, hildon/, and mimetypes/, and the personal menu config tool has options for selecting any of those. I think the gtk-update-icon-cache was all I needed: